This invention relates to the field of polymeric films, and packages made therefrom, for packaging a variety of products, particularly food. More particularly, the invention relates to a method of achieving an easy-opening package made from a polymeric film.
Generally, polymeric or thermoplastic materials are difficult to tear because the materials are flexible and are usually comprised of randomly dispersed, long chains of molecules having relatively strong covalent carbon-to-carbon bonds.
One method of easing tearing is generally to stretch or orient the thermoplastic materials as they are being cast or blown. Stretching lines up the molecular chains in the stretched direction so that if tearing occurs in that direction, it takes place through weaker non-carbon-to-carbon bonds. This method has limited effectiveness because not all polymers can be readily oriented. Also, only a certain amount of molecules can be aligned, and, despite the stretching, tearing may still not be easy and reliable. Orienting also requires special manufacturing efforts and care.
A more effective method of creating an easy-opening tear is to notch the seal of the package to initiate the tear. Notches may be created on the seal during the sealing operations by use of a seal jaw. Alternatively, notches may be created in the bag, at seal or otherwise, by use of a blade or another sharp device, during manufacture or packaging.
A significant problem with bags or films having a notch is that the notch is not precisely located or is disturbed during manufacture, packaging, transportation or general handling.
Another method of forming a tear line, widely used for single layer films, is to employ a star wheel to form a line of intermittent perforations in the films. Perforations are unsuitable for multilayer laminates for many reasons. One such reason is that perforations in the film allow product-environment exchanges which defeat a purpose of using multilayer films, i.e., to protect an interiorly contained product by combining gas barrier properties of one material with moisture barrier properties of another.
A third prior art method of imparting an easy tear opening to a laminate structure is the use of a tear tape or string. The tear tape or string is undesirable because it can result in structures being opened when the tear tape or string is ripped or otherwise disturbed during manufacture, packaging, transportation or handling.
A fourth prior art method of imparting an easy tear opening to a laminate structure is through the use of laser scoring. Laser scoring, however, requires the use of specialized equipment, and therefore, leads to increased costs of manufacture.
An alternative to the above four methods of producing an easy-opening effect is to "roughen," "knurl" or "knick" a designated area of film to create the easy-opening effect.
"Roughening or "knurling" the surface of a polymeric film may be done by various methods. The roughening can be formed by rotating a round abrasive such as a grindstone or wire brush, or other instrument having bristles, and putting the rotating instrument into contact with the film. Alternatively, a flat instrument having a roughened or grainy surface, such as for example a grain-containing sheet material, can be put into contact with the polymeric film.
The following patents and or publications, which teach easy-opening features of the type discussed above, are cited as references:
U.S. Pat. No. 2,703,764, to Vogt, discloses a thermoplastic tape with a weakened embrittled edge to facilitate tearing along the edge.
U.S. Pat. No. 3,608,815, to Bunch, teaches a pattern of porelike punctures over a fold line on the edge of a formed package.
U.S. Pat. No. 4,543,279, to Kai, teaches a film with an easy-opening feature of a plurality of random cuts or scratches formed along the edges of a film.
U.S. Pat. No. 5,141,795, also to Kai, teaches a pouch or package with an easy-opening feature of a multiplicity of through cuts formed in the edge portion of the package, wherein the through cuts are partly or wholly closed with the heat seal layer.
U.S. Pat. No. 4,778,058, to Yamazaki, teaches an easy-opening feature of a surface roughened portion of a film or pouch, wherein the surface roughened portion is positioned at and around the folded portion and at an external surface of the pouch.
European Patent Application No. 0 596 747 A1 teaches an easy-opening feature of a plurality of rows of linearly extending, laterally spaced slits formed parallel to and inwardly adjacent a side of the film.
It has now been found that an effective and practical method of creating an easy-opening package made of polymeric or flexible materials is to roughen or knurl the entire surface of the structure by applying a series of overall nicks or minute cuts over substantially the entire area of the outer layer of the structure.
The method of the invention creates an effective easy-opening package that has decreased costs of production over prior art methods. Since the entire surface area of the package is roughened or knurled, the easy-opening feature does not need to be registered to printing, or to the shape or structure of the final package, resulting in manufacturing efficiencies. This contrasts with prior art packages having easy-opening features, in which the easy opening feature is fixed in a particular location with relation to the printing or to the shape or structure of the finished package.
The method of the invention also creates a packaging material or package that does not have notches or tear strips but is rather a smooth surface. Thus, the laminate and packaging material of the invention avoid the problem of tearing or ripping during manufacture, packaging, transportation or general handling, that is common in the prior art.